The effects of environment and dwell on high temperature fatigue crack growth of 2 1/4 Cr - 1 Mo steel.
Kamen, John S.
McNelley, Terry R.
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The effects of the environment and dwell periods on fatigue crack growth rates at 525°C for both annealed and normalized and tempered 2 1/4 Cr - 1 Mo steel in High Strain Fatigue (HSF) and Linear Elastic Fracture Mechanics (LEFM) regimes in air and in vacuum have been examined using optical and scanning electron microscopy techniques. Fatigue crack propagation rates were determined and were found to vary with the environment and the loading waveform used in the tests. At small crack depths in HSF, oxidation in air increased cyclic crack growth rates by over an order of magnitude compared with vacuum. For continuous cycling tests at a frequency of 0.01 Hz, and tests with a peak tension, compression, or tension plus compression dwell cycling, the rates were similar. The fastest crack growth in air occurred during cycles having both tension and compression dwell periods. Fractographs were analyzed in attempts to understand the reasons for the different propagation rates. Evidence suggesting partial rewelding of the crack surfaces during compressive dwell periods in vacuum and a change in fracture mode from transgranular to a branching "intergranular-like" fracture when the environment was changed from air to vacuum were observed.
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